Plasma arc welding, for example, plasma arc spot welding is being utilized in a variety of fields. This is because plasma arc welding enables metal workpieces laid one on top of the other to be welded together from one side, which is advantageous in that it enables the welding of an intricately-shaped workpiece, and a large workpiece.
However, conventional plasma arc welding proves problematic when welding a metal coated on a surface with a substance that has a lower melting point than a melting point of a base metal, such as galvanized sheet steel, in that satisfactory welding results cannot be obtained because a surface coating material vaporizes, and a vapor thereof penetrates a molten pool of a weld causing an explosion, and/or leaving a hole in a weld.
To solve for this problem, the applicant of the present case proposed in Japanese Patent Application No. 6-061138 a welding method, wherein 2 steps of vapor escape and main welding are performed consecutively by adjusting the power of a plasma arc. That is, initially, using a relatively high power plasma arc, a through-hole is bored in at least one of the steel sheets to be welded together, and a vapor of a coating material is allowed to escape via this through-hole. Next, the power of a plasma arc is lowered to an appropriate level, and two steel sheets are welded.
In the welding method of Japanese Patent Application No. 6-061138, when there is a gap between 2 steel sheets to be welded, because a molten metal is expended in the gap, there are cases in which a vapor escape hole cannot be flattened by subsequent welding steps, or high weld strength cannot be achieved, or, when a gap is large, welding itself cannot be performed. According to a test of the inventors, it was determined that welding substantially cannot be performed when a gap between steel sheets exceeds around 0.2mm.
Therefore, an object of the present invention is, in plasma arc welding, to make it possible to achieve satisfactory welding even when there is a gap between welding materials.
An additional object of the present invention is, in plasma arc welding, to make a vapor of a low-boiling-point material coating a welding material effectively escape, and to make it possible to achieve satisfactory welding even when there is a gap between welding materials.
And yet an additional object of the present invention is, in plasma arc welding, make it possible to measure a gap between welding materials automatically.